The present invention relates to apparatuses and methods for use in performing spinal surgery and, in particular, to flexible bone attachment structures or implants for spinal support and alignment that provide variable degrees of segmental stiffness, and therefore flexibility, preferably using minimally or less invasive techniques for insertion of the implants. Due to the different degrees of segmental stiffness, certain embodiments of the apparatus of the present invention can be used with and/or without fusion.
The spine is structured as a repeating sequence of vertebrae, intervertebral discs and facet joints supported and held together by surrounding ligaments and muscles. The vertebra is a block of bone configured as a body anteriorly and laminae, extending posteriorly to form a spinous process, which are connected in the middle by a pair of pedicles. The spine can be divided into motion segments which include two adjacent vertebrae anteriorly, an intervening disc and associated facet joints posteriorly. The spine can be bent, compressed, stretched and twisted. In certain alignments, the spine is fairly shear resistant, but in some alignments it is not shear resistant. The spine is construed as a column which can be divided into three sections: anterior, middle and posterior columns. The anterior column includes the front half of the discs; the middle column includes the back half of the discs, plus the spinal canal and pedicles; and the posterior column includes the facet joints, laminae and spinous processes. The spine is thus a continuation of connected articulated motion segments which can be bent in multiple directions, including flexion and extension. Natural or normal bio-mechanical movement of the spine requires shortening of the posterior column length in extension and elongation or expansion of this length in flexion, with a substantial change in the interpedicular distance.
Historically, it has been common to fuse adjacent vertebrae that are placed in fixed relation by the insertion or installation therealong of bone screws or other bone anchors and cooperating longitudinal connecting members or other elongate members. Fusion results in the permanent and very stiff immobilization of one or more of the spinal motion segment intervertebral joints. Because the anchoring of bone screws, hooks and other types of anchors directly to a vertebra can result in significant forces being placed on the vertebra, and such forces may ultimately over time result in the loosening of the bone screw or other anchor implants from the vertebra due, in part, to the considerable stiffness of such implants, fusion allows for the growth and development of a permanent bone counterpart to the longitudinal connecting member that can maintain the spine in the desired position, even if the implants ultimately fail, fracture, loosen or are removed. However, fusion itself also results in considerable stiffness of the spinal segment being fused with its own associated consequences. Because fusion has been a desired component of spinal stabilization procedures in the past, longitudinal connecting members have been designed that are of a material, size and shape to largely resist flexion, extension, torsion, side bending, distraction and compression, and thus substantially immobilize the portion of the spine that is to be fused. Thus, longitudinal connecting members are typically uniform along an entire length thereof, and usually made from a single or integral piece of material having a uniform diameter, width or cross-sectional area of a size to provide substantially firm rigid support in all planes with little flexibility. Again, fusion often results in too much stiffness for the segment of spine being fused, even if the implants are later removed. This can result in multiple adverse side-effects, including loss of motion and accelerated degenerative changes at junctional levels.
An alternative to fusion, which immobilizes at least a portion of the spine, and the use of more stiff and even rigid longitudinal connecting members or other stiff, rigid and hard structures has been a “soft” or “dynamic” stabilization approach in which a flexible loop-, S-, C- or U-shaped member or a coil-like and/or a spring-like member is utilized as a less stiff longitudinal connecting member with elastic fixed return between a pair of pedicle screws in an attempt to create a flexible stabilization and the possibility for a more normal loading pattern between the vertebrae in flexion, extension, distraction, compression, side bending and torsion. Another type of soft or less stiff system known in the art includes bone anchors connected by limp cords or strands that can be bent and that intrinsically have little to no bending stiffness. Such a cord or strand may be threaded through cannulated spacers that are disposed between adjacent bone anchors when such a cord or strand is implanted, tensioned and attached to the bone anchors, thereby compressing the spacers. The spacers typically span the distance between bone anchors, providing some degree of bending stiffness and limits on the bending movement of the surrounded cord or strand and thus strengthening and supporting the overall system. However, such known systems have provided limited control with respect to torsional and shear forces and little to no allowance for lengthening or increasing distance between the heads or bodies of posteriorly positioned bone anchors with flexion, which is critical for correct spinal bio-mechanics as it relates to flexible stabilization or even to natural segmental spinal motion. Also, such known systems have provided no differentiation between bending stiffness in flexion compared to that in extension (i.e., more stiffness in flexion versus that in extension).